Constant frequency difference heterodyne



March 6, 1934. w. VAN B. ROBERTS 1,949,843

CONSTANT FREQUENCY DIFFERENCE HETERODYNE Filed Sept. 17, 1929 1 1 4\mvio #4010 060.

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INVENTOR WALTER VAN EROBERTS BY ATTORNEY Patented Mar. 6, 1934 PATENTOFFICE CONSTANT FREQUENCY DIFFERENCE HETERODYNE Walter van B. Roberts,Princeton, N. 1., assignor to Radio Corporation of America, acorporation of Delaware Application September 17, 1929, Serial No.393,142

Claims.

My present invention relates to heterodyne circuits, and moreparticularly, to a superheterodyne circuit having a constantintermediate or difference frequency.

One of the most diflicult problems encountered in unicontrolsuperheterodyne practice is that of maintaining constancy ofintermediate or difference frequency. While it is true that severalmethods have been suggested in the past for securing a constantfrequency difierence in a superheterodyne circuit, yet these priormethods have not been ideal in their construction and performance.

For example, unicontrolled, straight line frequency gang condensers havebeen employed in the signal and local oscillator circuits ofsuperheterodyne receivers. However, this solution necessitates thestraight line frequency type of tuning which is not popular at present,since most of the best liked stations operate on the lower frequenciesof the broadcast band, and are too much crowded into a small portion ofthe dial where straight line frequency tuning is employed. Also straightline frequency condensers are more bulky and difiicult to manufacturethan condensers such as are in common use today.

Again, attempts have been made to employ unicontrol gang condensers, thecondensers all having the same shapes. Since it was extremely desirableto maintain this gang construction, electrical means of various typeswere employed to avoid the necessity of offsetting the oscillatorcondenser relative to the remaining condensers. These additionalelectrical means, however, were difficult to adjust, and could onlyresult, at best, in an approximation to the desired results.

Now, I have invented a tuning arrangement for superheterodyne receiversin which all the benefits of unicontrol; gang condenser construction canbe retained and at the same time secure the usual and full 180adjustment of the gang condensers, without recourse being had to anyexpensive or highly complicated electrical means independent ofunicontrol means. That is to say, my invention provides for anadjustment to secure a constant frequency difference while adjusting theunicontrol means itself. Additionally, the nature of the means proposedhereinafter, is such that the mechanical structure of present daysuperheterodyne receivers employing unicontrol, gang condensers, thegang condensers being disposed in the radio frequency circuits andoscillator circuit, need not be serious-' ly altered to embody myinvention.

.' Accordingly, it is one of the main objects of:

the present invention to provide a superheterodyne receiver embodying aunicontrol tuning means, the latter including a gang of condensershaving electrodes of any desired shape, the unicontrol means,additionally, including an adjusting member operative simultaneouslywith the unicontrol means to maintain a constant intermediate ordifference frequency.

' Another important object of the invention is to provide in asuperheterodyne receiver a unicontrolled, gang condenser means adaptedto maintain a constant difference frequency, the unicontrol meansembodying an additional means, the latter being simultaneously operativewith the unicontrol means and comprising a variable capacity of apredetermined magnitude and having an electrode of predetermined shape,the gang condensers including electrodes of any desired shape.

Other objects of the invention are to improve generally the simplicityand efficiency of unicontrol tuning means for superheterodyne receivers,and to provide a device of this nature which is durable and reliable inoperation, economical to manufacture, and, above all, readily able tomaintain an absolutely constant frequency difference. I

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing,

Fig. 1 schematically shows a superheterodyne receiving circuit ofpresent day construction,

Fig. 2 is a detailed view of a portion of the circuit, shown in Fig. l,embodying one form of my invention,

Fig. 3 is a detailed view similar to Fig. 2, and shows another form ofthe invention,

Fig. 4 is a. detailed view similar to Figs. 2 and 3 and shows thepreferred form of the invention,

Fig. 5 shows one form of a specially shaped plate embodied in theinvention.

Referring to the accompanying drawing in which like characters ofreference indicate the same parts in the different figures, Fig. 1schematically shows a well-known type of superheterodyne receivingcircuit, in which a grounded antenna circuit A, G is connected to atuned radio frequency amplifier 1, the output circuit of the amplifiercomprising a tunable, signal circuit S. Circuit S includes an inductanceL and a variable tuning condenser C.

A local oscillator 2 impresses its oscillations upon the tuned inputcircuit S1 of the first detector 3. The output circuit of the localoscillator 2 comprises a tunable oscillator circuit 0 which includes aninductance L1 and a variable capacity 01. The input circuit S1 of thedetector 3 includes an inductance L2 and a variable capacity C2.

The output of the first detector is amplified by means of anintermediate frequency amplifier 4, the output of the latter beingpassed through a second detector 5. The output of the latter is utilizedin any well-known manner as by phones, loud-speakers, etc.

The variable capacities C, C1, C2 are unicontrolled in the well-knownand usual manner. That is to say, the rotors of these capacities aremounted in gang fashion upon a single, rotatable shaft. As explainedheretofore, these three condensers should preferably be of the sameshape and of a shape readily produced. Furthermore, for the purposes ofmy invention, it is highly desirable that these three condensers be ofthe type universally known in the United States as the bath-tub gangcondenser, so that no expensive or extensive change must be made inpresent day receiver practice to embody the herein disclosed means. Thistype of gang condenser comprises a common metallic housing having aU-shaped cross-section, the rotor shaft being supported at the oppositeends of the housing and the stators of the condenser units being securedto the sides and bottom of the housing.

In Fig. 2, I have shown a portion of the superheterodyne receivingcircuit, as shown in Fig. 1. That is to say I have shown the signalcircuits S and S1 and the oscillator circuit 0. The signal circuit S, asshown in Fig. 1, is connected to the amplifier 1. The oscillator circuit0 is connected to the oscillator tube while the circuit S1 is connectedto the detector 3.

In parallel with the variable capacity C1 in the oscillator circuit 0there is disposed a fixed capacity Co. The three condensers C. C1, C2,are mounted on a single shaft, and unicontrolled, as in Fig. 1. By thesuitable choice of the fixed capacity Co in parallel with the condenserC1 and of L1, it is possible to obtain a fair degree of constancy offrequency difference over a portion of the broadcast range. It can beshown from purely theoretical considerations that, for absoluteconstancy of beat frequency, the capacity in parallel with the variablecondenser where, n=21r intermediate frequency=nons. Co may be computedfor each value of 98. values of L and L1 being assumed. For example,assume L=250; experimenting with the ratio L I; 1.62 we find Co runsfrom zero at the low frequency end, up to 23.9 and then down to 15.2 at1500 kilocycles. If

L -1119 Cv runs from 20.8 to 35 and then down to 19.

It may be pointed out here that this capacity Co can be obtained byhaving the rotor of the gang condenser pass a specially shaped platesecured to and connected to the stator. If the end rotor plate liesoutside the stator, an extra plate will accomplish this purpose.

In Fig. 3 is shown a fixed condenser Co in series with C1, which alsogives a certain degree of constancy of beat frequency. The constancy maybe made absolute in this case, also, by adding an extra, speciallyshaped stator plate to C1.

In Fig. 4 there is shown the preferred form of the invention. Thecircuits S, S1 and O include the same tuning condensers C, C1, C2 andinductances L, L1, L2, as shown in the other figures. The oscillatorcircuit 0 has disposed therein a small, fixed capacity K in shunt withthe inductance L1, while a small variable capacity Cu is connected inparallel with the main tuning condenser C1, the variable capacity Cvbeing arranged for unicontrol with the main condenser. By proper choiceof the fixed capacity K and the inductance L1 the signal frequency andthe superheterodyne frequency difference will be very nearly a constantamount over the entire broadcasting range.

However, to maintain the difierence frequency exactly constant, thesmall condenser C is employed, the rotor or stator plates of whichcondenser are specially shaped, the condenser being in parallel with theinductance L1, and unicontrolled with the main condenser C1. The shapeof the plates of the capacity C can readily be determined by anyoneskilled in the art, one such shape being shown merely by way ofillustration, in Fig. 5, it being clearly understood that the presentinvention is not limited to any such shape.

Thus, in order to adapt my invention to present superheterodynereceivers, it is merely necessary to secure a condenser embodying aspecially shaped plate as shown in Fig. 5 to the unicontrol tuning shaftnow employed in the aforesaid receivers, the small fixed capacity Kbeing connected in shunt with the oscillator inductance L1, the saidinductance being changed slightly to conform with the value chosen forthe fixed capacity K.

This change, as can be readily seen, involves no expensive or extensivechanges in the present receivers, and does not require any change in themethod of manufacture of superheterodyne receivers. It will also beunderstood that the elements required by my invention are inexpensive.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications in the circuitarrangements, as well as in the apparatus employed, may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. A superheterodyne receiving system comprising a signal circuit and anoscillator circuit, each of said circuits being provided with a variablecondenser of the same characteristics, said variable condensers havingvariable elements mounted upon a common varying means, a fixed capacityin parallel with the variable condenser in the oscillator circuit, and avariable condenser in parallel with said oscillator tuning condenser,

said variable condenser being uni-controlled with said common varyingmeans and being so designed that it maintains a constant frequencydifference between the oscillator and signal circuits.

2. In a superheterodyne receiver, a signal circuit including a variablecondenser, an oscillator circuit including a variable condenser, acommon control for simultaneously and similarly varying the rotors ofsaid condensers, a capacity connected in said oscillator circuit formaintaining the difference frequency between said signal and oscillatorcircuits approximately constant over the entire broadcast range, and anadditional variable condenser connected in shunt to said oscillatorvariable condenser, and arranged for uni-control with said variablecondensers, for maintaining said difference frequency exactly constantover said range.

3. In a superheterodyne receiver, a signal circuit including a variablecondenser, an oscillator circuit including a variable condenser of thesame type, a common control for simultaneously and similarly varying therotors of said condensers, and an additional variable condenser,connected across the variable condenser in said oscillator circuit, andarranged for uni-control with said variable condensers for maintainingthe difference frequency between said signal and oscillator circuitssubstantially constant over the entire broadcast range.

4. In a superheterodyne receiver, a signal circuit including a tuningdevice having an adjustable element, an oscillator circuit including asecond tuning device of the same type having an adjustable element, acommon control for simultaneously and similarly varying the adjustableelements of both tuning devices, said circuits being tunable throughdifferent frequency ranges, and an additional adjustable tuning device,connected across the tuning device in said oscillator circuit, arrangedfor uni-control adjustment with said first two tuning devices formaintaining the difference frequency between said circuits substantiallyconstant through said ranges.

5. In a superheterodyne receiver, a signal circuit including a variablecondenser, an oscillator circuit including a variable condenser, acommon control for simultaneously and similarly varying the rotors ofsaid condensers, and an additional variable condenser connected in saidoscillator circuit and arranged for uni-control with said variablecondensers for maintaining the difference frequency between said signaland oscillator circuits substantially constant over the entire broadcastrange, said additional variable condenser being connected in parallelwith the oscillator circuit variable condenser.

6. In a superheterodyne receiver, a signal circuit including a tuningdevice having an adjustable element, an oscillator circuit including asecond tuning device having an adjustable element, a common control forsimultaneously and similarly varying the adjustable elements of bothtuning devices, said circuits being tunable through different frequencyranges, and an additional adjustable tuning device, in said oscillatorcircuit, arranged for uni-control adjustment with said first two tuningdevices for maintaining the difference frequency between said circuitssubstantially constant through said ranges, said additional adjustabletuning device being connected in shunt with said oscillator circuittuning device.

7. In a superheterodyne receiver, a signal circuit including a variablecondenser, an oscillator circuit including a variable condenser, acommon control for simultaneously and similarly varying the rotors ofsaid condensers, and an additional variable condenser connected inparallel with the variable condenser of said oscillator circuit andarranged for uni-control with said variable condensers for maintainingthe difference frequency between said signal and oscillator circuitssubstantially constant over the entire broadcast range, said additionalvariable condenser including at least one electrode of a predeterminedshape.

8. In a superheterodyne receiver, a signal circuit including a variablecondenser, an oscillator circuit including a variable condenser, acommon control for simultaneously and similarly varying the rotors ofsaid condensers, and an additional variable condenser connected in shuntwith the variable condenser of said oscillator circuit and arranged foruni-control with said variable condensers for maintaining the differencefrequency between said signal and oscillator circuits substantiallyconstant over the entire broadcast range, said additional variablecondenser including at least one rotor element of predetermined shape.

. 9. In an electrical apparatus, the combination 10. In asuperheterodyne receiver, of a type comprising a signal circuitincluding a coil and a shunt variable tuning condenser, a localoscillation circuit including a coil and a shunt variable tuningcondenser of the same type as the first, and a common adjustable controldevice for the rotors of both condensers, means for maintaining theintermediate frequency difference between the signal and oscillatorcircuits exactly constant throughout the receiver tuning rangecomprising a small fixed condenser in shunt with the oscillatorcondenser, and a small variable capacity in shunt with the oscillatorcoil, the rotor of the last capacity being arranged on said controldevice for adjustment thereby.

